Electrical stimulation technology is expanding in scope, resulting in therapies that place leads in more and more areas of the body and in leads that have increasing levels of complexity. The stiffness of a lead body can be important for its function in many ways. For example, a very flexible lead body may be desirable for implantation in tissues that experience a lot of movement. A stiffer lead body may be important for guiding through tissue or for insertion into a device, such as an electrical signal generator. For example, if the portion of a lead to be inserted into a device is not sufficiently stiff, pushing the lead into a receptacle of the device may be difficult.
In transitioning from a stiff to a more flexible segment along the length of a lead, it is desirable for the transition to be smooth to spread any stresses out evenly over the length of the transition. Many ways of transitioning between segments of differing lead flexibilities have been devised. However, some are impractical from a manufacturing perspective or result in undesired characteristics. For example, lap bonding of two sections of differing stiffnesses can result in a reliable bond that spreads stresses out over the length of the lap bonded section; however, such lap bonding also results in increased thickness of the lead body. Such leads have a bulge in their outer diameter, which may be undesirable.